There are two switches which start the aux fan.(the one in front of the radiator) The water temp switch is bottom right of radiator(facing engine). Also a pressure switch on the ac drier will turn it on. So anytime your ac is on the fan is on. Hope this helps.

Yeah, it's more than likely that temp switch mounted on the Drier or the one on the Radiator?

RJM65 showed me a way to bypass the Drier switch and run a Relay to the Air Valve mounted on the crossbar. Now the fan always kicks in when the AC is turned on and not when the Drier Switch decides to.

The factory fan works great, if the car is heating up, something is wrong.

Most common problems are;

Silicone oil has leaked out of the fan clutch so it isn't locking up when hot.
Radiator is full of scale and junk.
Engine is out of tune, too lean, etc. and making too much heat.
Bad thermostat.
Bubble in the cooling system.

The Aux fan is designed to only come on when the AC is running or the engine is about to overheat, so not running during normal driving is normal.

FF,
Danglerb has given you some good ideas.
The first one I would try is to 'burp' your system. Just grab the top radiator hose and squeeze with the resevoir top open. You will be surprised how much air will come out and for how long. I have gotten up to an additional gallon of distilled water into my system by burping it. In the overall scheme of things, that is quite a bit of additional cooling capacity, and it could be the cheapest fix. (free)

Next would be the radiator. You can't have the radiator rodded because of the design, BUT you can have it vatted and flushed by a radiator shop. I use a local radiator shop that is familiar with Behr plastic ended radiators. They have a jig to hold the radiator and do not damage the tabs that hold the plastic end caps on. If I am replacing an end cap, I buy it and bring it with me. (Normally $65-$95 for the radiator work + @$100 cost per end cap as necessary)

For the viscous fan, just turn the fan while the engine is cold, and note the pressure it takes to turn the fan. Then, get the engine hot, turn the fan, and note the pressure. It should be harder to turn when hot. If it's not, you can buy the silicone in a squirt tube to put back in there. Never had to do this, so I cannot tell you the price, but I believe its a Toyota part.

Hey firefighter7128, One other circuit upgrade that I added to Miss Purdy (84 - 928) was to wire in a toggle switch as a 3rd control option to turn on the auxiliary electric radiator fan. Normally the electrical aux fan relay (XVIII) receives a ground from either the large radiator temp sense switch (bottom front of radiator) or the smaller A/C refrigerant temp switch (front of condenser – screwed into a heat sink on the top side of the freon drier – the switch on the front of the drier is only for low freon pressure sense)

By soldering in a toggle switch in parallel with these two temp sensing switches you can manually select the electric fan to come on early if stuck in traffic and you begin to see your engine temp gauge creeping upward. I ran a couple of wires from the back side of the relay XVIII socket to the rear hatch release panel on the left side of the driver’s seat to mount the switch. The toggle switch has one wire connected to pin 85 of the relay plug-in socket and the other wire goes to one of the primary ground points above the fuse panel.